When you get your teeth whitened, you don’t expect to end up with anything more than a dazzling smile. But Abbie Kilbride, an 18-year-old from the U.K., found herself with swollen lips, blisters, and third-degree burns in her mouth.

Kilbride wrote about and shared photos of her experience in a Facebook post that’s gone viral. According to her post and an interview with the Daily Record, she visited a woman several of her friends had used who did teeth whitening out of her house. The procedure took an hour and every 10 to 15 minutes, the woman would add more gel to Kilbride's teeth. It was only when the process was done that Kilbride noticed something was off.

“After I was finished, my whole mouth was numb,” she writes on Facebook. “I just assumed it was because I had a gum shield in my mouth for over an hour, but when I looked in the mirror, my lips were quite swollen and I was struggling to talk…About two hours later, after I used ice and an antihistamine, it had went down a fair bit and [I] thought that would be the end of it.”

But the next morning, she woke up to find that her lips were even more swollen and stuck together. She saw a doctor, who prescribed an antihistamine, but the next morning she had two huge blisters in her mouth. “I couldn't open my mouth to speak [or] eat as it felt like my gums were about to rip apart,” she wrote. She eventually visited a dentist, who told her she had suffered third-degree burns in her mouth, and a possible allergic reaction to the chemicals used. Now, she says, she’s on better medication and a strict diet of soup and scrambled eggs “as anything else is too painful.”

While Kilbride’s experience is freaky, experts say it’s not normal to experience side effects like this if you have your teeth whitened by a professional. “This is very uncommon,” Susan Maples, D.D.S., author of Blabber Mouth! 77 Secrets Only Your Mouth Can Tell You to Live a Healthier, Happier, Sexier Life, tells SELF. “It makes me wonder what the product was.”

Mark S. Wolff, D.D.S., Ph.D., a professor and chair of the Department of Cariology and Comprehensive Care at the New York University College of Dentistry, tells SELF that he’s seen burns like this in his practice—from a patient who tried to whiten her teeth using Clorox bleach.

While Wolff says it’s rare that you’ll develop complications from teeth whitening done at the dentist, he points out that the chemicals used by some dentists are “extremely concentrated.” “You have to be very careful that it doesn’t get on the tongue, lips, or gums, since it can cause burns,” he says.

But Martha Cortes, D.D.S., of Cortes Advanced Dentistry, tells SELF that a person can experience swollen lips if the teeth-whitening product is left on for too long. (This can also make teeth look more transparent, glassy, and even grey-ish in color, she adds.)

It also depends on the type of teeth-whitening treatment you have done, Maples says—she normally uses a version of carbonate peroxide. “Typically we only notice sensitivity of teeth after the procedure, and that goes away after awhile,” she says. Maples has noticed that some over-the-counter teeth-whitening strips can cause gum irritation, but says she’s “never seen or heard of” someone having as severe a reaction as Kilbride, who appears to have had contact dermatitis—an allergic reaction to a chemical.

Adam Barsoum, D.M.D., of Boston Dental, tells SELF that it's also possible Kilbride had a reaction to the latex in the rubber dam used on her teeth—something to keep in mind if you have a latex allergy and want to get your teeth whitened.

Overall, experts stress the importance of visiting your dentist if you want to have your teeth whitened properly and safely. “We can control the amount of product and treat any sensitivity that may arise,” Maples says.

“This is normally very safe to do,” Wolff notes. “You don’t need to be worried about it, as long as you visit a professional.”